248 lines
10 KiB
Markdown
248 lines
10 KiB
Markdown
---
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c: Copyright (C) Daniel Stenberg, <daniel.se>, et al.
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SPDX-License-Identifier: curl
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Title: libcurl
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Section: 3
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Source: libcurl
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See-also:
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- libcurl-easy (3)
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- libcurl-multi (3)
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- libcurl-security (3)
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- libcurl-thread (3)
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---
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# NAME
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libcurl - client-side URL transfers
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# DESCRIPTION
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This is a short overview on how to use libcurl in your C programs. There are
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specific man pages for each function mentioned in here. See
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libcurl-easy(3), libcurl-multi(3), libcurl-share(3),
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libcurl-url(3), libcurl-ws(3) and libcurl-tutorial(3) for
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in-depth understanding on how to program with libcurl.
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There are many bindings available that bring libcurl access to your favorite
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language. Look elsewhere for documentation on those.
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# TRANSFERS
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To transfer files, you create an "easy handle" using curl_easy_init(3)
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for a single individual transfer (in either direction). You then set your
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desired set of options in that handle with curl_easy_setopt(3). Options
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you set with curl_easy_setopt(3) stick. They are then used for every
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repeated use of this handle until you either change the option, or you reset
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them all with curl_easy_reset(3).
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To actually transfer data you have the option of using the "easy" interface,
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or the "multi" interface.
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The easy interface is a synchronous interface with which you call
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curl_easy_perform(3) and let it perform the transfer. When it is
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completed, the function returns and you can continue. More details are found in
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the libcurl-easy(3) man page.
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The multi interface on the other hand is an asynchronous interface, that you
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call and that performs only a little piece of the transfer on each invoke. It
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is perfect if you want to do things while the transfer is in progress, or
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similar. The multi interface allows you to select() on libcurl action, and
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even to easily download multiple files simultaneously using a single
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thread. See further details in the libcurl-multi(3) man page.
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# SUPPORT INTERFACES
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There is also a series of other helpful functions and interface families to
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use, including these:
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## curl_version_info()
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gets detailed libcurl (and other used libraries) version info. See
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curl_version_info(3)
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## curl_getdate()
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converts a date string to time_t. See curl_getdate(3)
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## curl_easy_getinfo()
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get information about a performed transfer. See curl_easy_getinfo(3)
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## curl_mime_addpart()
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helps building an HTTP form POST. See curl_mime_addpart(3)
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## curl_slist_append()
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builds a linked list. See curl_slist_append(3)
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## Sharing data between transfers
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You can have multiple easy handles share certain data, even if they are used
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in different threads. This magic is setup using the share interface, as
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described in the libcurl-share(3) man page.
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## URL Parsing
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URL parsing and manipulations. See libcurl-url(3)
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## WebSocket communication
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See libcurl-ws(3)
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# LINKING WITH LIBCURL
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On unix-like machines, there is a tool named curl-config that gets installed
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with the rest of the curl stuff when 'make install' is performed.
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curl-config is added to make it easier for applications to link with libcurl
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and developers to learn about libcurl and how to use it.
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Run 'curl-config --libs' to get the (additional) linker options you need to
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link with the particular version of libcurl you have installed. See the
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*curl-config(1)* man page for further details.
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Unix-like operating system that ship libcurl as part of their distributions
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often do not provide the curl-config tool, but simply install the library and
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headers in the common path for this purpose.
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Many Linux and similar systems use pkg-config to provide build and link
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options about libraries and libcurl supports that as well.
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# LIBCURL SYMBOL NAMES
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All public functions in the libcurl interface are prefixed with 'curl_' (with
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a lowercase c). You can find other functions in the library source code, but
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other prefixes indicate that the functions are private and may change without
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further notice in the next release.
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Only use documented functions and functionality!
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# PORTABILITY
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libcurl works
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**exactly**
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the same, on any of the platforms it compiles and builds on.
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# THREADS
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libcurl is thread safe but there are a few exceptions. Refer to
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libcurl-thread(3) for more information.
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# PERSISTENT CONNECTIONS
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Persistent connections means that libcurl can reuse the same connection for
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several transfers, if the conditions are right.
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libcurl always attempts to use persistent connections. Whenever you use
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curl_easy_perform(3) or curl_multi_perform(3) etc, libcurl
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attempts to use an existing connection to do the transfer, and if none exists
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it opens a new one that is subject for reuse on a possible following call to
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curl_easy_perform(3) or curl_multi_perform(3).
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To allow libcurl to take full advantage of persistent connections, you should
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do as many of your file transfers as possible using the same handle.
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If you use the easy interface, and you call curl_easy_cleanup(3), all
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the possibly open connections held by libcurl are closed and forgotten.
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When you have created a multi handle and are using the multi interface, the
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connection pool is instead kept in the multi handle so closing and creating
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new easy handles to do transfers do not affect them. Instead all added easy
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handles can take advantage of the single shared pool.
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# GLOBAL CONSTANTS
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There are a variety of constants that libcurl uses, mainly through its
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internal use of other libraries, which are too complicated for the
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library loader to set up. Therefore, a program must call a library
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function after the program is loaded and running to finish setting up
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the library code. For example, when libcurl is built for SSL
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capability via the GNU TLS library, there is an elaborate tree inside
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that library that describes the SSL protocol.
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curl_global_init(3) is the function that you must call. This may
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allocate resources (e.g. the memory for the GNU TLS tree mentioned above), so
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the companion function curl_global_cleanup(3) releases them.
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If libcurl was compiled with support for multiple SSL backends, the function
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curl_global_sslset(3) can be called before curl_global_init(3)
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to select the active SSL backend.
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The global constant functions are thread-safe since libcurl 7.84.0 if
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curl_version_info(3) has the CURL_VERSION_THREADSAFE feature bit set
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(most platforms). Read libcurl-thread(3) for thread safety guidelines.
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If the global constant functions are *not thread safe*, then you must
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not call them when any other thread in the program is running. It
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is not good enough that no other thread is using libcurl at the time,
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because these functions internally call similar functions of other
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libraries, and those functions are similarly thread-unsafe. You cannot
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generally know what these libraries are, or whether other threads are
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using them.
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If the global constant functions are *not thread safe*, then the basic rule
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for constructing a program that uses libcurl is this: Call
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curl_global_init(3), with a *CURL_GLOBAL_ALL* argument, immediately
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after the program starts, while it is still only one thread and before it uses
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libcurl at all. Call curl_global_cleanup(3) immediately before the
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program exits, when the program is again only one thread and after its last
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use of libcurl.
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It is not actually required that the functions be called at the beginning
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and end of the program -- that is just usually the easiest way to do it.
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You can call both of these multiple times, as long as all calls meet
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these requirements and the number of calls to each is the same.
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The global constant situation merits special consideration when the code you
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are writing to use libcurl is not the main program, but rather a modular piece
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of a program, e.g. another library. As a module, your code does not know about
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other parts of the program -- it does not know whether they use libcurl or
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not. Its code does not necessarily run at the start and end of the whole
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program.
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A module like this must have global constant functions of its own, just like
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curl_global_init(3) and curl_global_cleanup(3). The module thus
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has control at the beginning and end of the program and has a place to call
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the libcurl functions. If multiple modules in the program use libcurl, they
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all separately call the libcurl functions, and that is OK because only the
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first curl_global_init(3) and the last curl_global_cleanup(3) in a
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program change anything. (libcurl uses a reference count in static memory).
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In a C++ module, it is common to deal with the global constant situation by
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defining a special class that represents the global constant environment of
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the module. A program always has exactly one object of the class, in static
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storage. That way, the program automatically calls the constructor of the
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object as the program starts up and the destructor as it terminates. As the
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author of this libcurl-using module, you can make the constructor call
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curl_global_init(3) and the destructor call curl_global_cleanup(3)
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and satisfy libcurl's requirements without your user having to think about it.
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(Caveat: If you are initializing libcurl from a Windows DLL you should not
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initialize it from *DllMain* or a static initializer because Windows holds
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the loader lock during that time and it could cause a deadlock.)
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curl_global_init(3) has an argument that tells what particular parts of
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the global constant environment to set up. In order to successfully use any
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value except *CURL_GLOBAL_ALL* (which says to set up the whole thing), you
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must have specific knowledge of internal workings of libcurl and all other
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parts of the program of which it is part.
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A special part of the global constant environment is the identity of the
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memory allocator. curl_global_init(3) selects the system default memory
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allocator, but you can use curl_global_init_mem(3) to supply one of your
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own. However, there is no way to use curl_global_init_mem(3) in a
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modular program -- all modules in the program that might use libcurl would
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have to agree on one allocator.
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There is a failsafe in libcurl that makes it usable in simple situations
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without you having to worry about the global constant environment at all:
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curl_easy_init(3) sets up the environment itself if it has not been done
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yet. The resources it acquires to do so get released by the operating system
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automatically when the program exits.
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This failsafe feature exists mainly for backward compatibility because there
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was a time when the global functions did not exist. Because it is sufficient
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only in the simplest of programs, it is not recommended for any program to
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rely on it.
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